<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>同心圆效果</title>
<style>
  .container::before,
  .container::after {
    content: '';
    position: absolute;
    top: 50%;
    left: 50%;
    width: 100px;
    height: 100px;
    border-radius: 50%;
    background: linear-gradient(90deg, #ff9, #9bff);
    transform: translate(-50%, -50%);
  }
  .container::before {
    border: 1px solid #333;
  }
  .container::after {
    background: linear-gradient(90deg, #9bff, #ff9);
  }
</style>
</head>
<body>
<div class="container"></div>
</body>
</html>